Preparation of Anhydrous Magnesium Chloride: Solid–Liquid Phase Diagram for the System MgCl₂–NH₃–C₂H₄[OH]₂ at 323 K

Abstract

The ammoniated magnesium chloride hexammoniate compound (HEX) is the key precursor phase required for the production of anhydrous magnesium chloride by the Australian Magnesium (AM) process. It is produced by direct ammoniation of MgCl2-saturated ethylene glycol solutions at 323 K. To determine the conditions required to form HEX, the C2H4[OH]2-rich part of the MgCl2–NH3–C2H4[OH]2 system was investigated at 323 ± 0.5 K. Seven phase regions were determined. These were: NH3(g)+LiqT, HEX+LiqT+NH3(g), HEX+LiqT, HEX+T+LiqT, T+LiqT, MgCl2·3EG+T+LiqT, and LiqT. The symbol T represents a ternary compound of composition MgCl2·2NH3·2C2H4[OH]2, and LiqT represents a ternary liquid phase. To produce only hexammoniate in the AM process, bulk ammonia levels need to be maintained at levels of greater than about (11 to 13) % (w/w) NH3. At lower ammonia levels, the formation of T-phase is promoted, resulting in coprecipitation of HEX and T-phase.

An Efficient, Solvent-Free Process for Synthesizing Anhydrous MgCl2

“…Finally, by 140 °C for an additional 35 min, strong evidence of the monohydrate was observed. In comparison with the dehydration profiles reported in the literature, the dehydration temperatures under N 2 flow are significantly lower. ,, …”

“…Multiple steps of bischofite (MgCl 2 ·6H 2 O) dehydration at elevated temperatures include , Also, at T = 180 °C partial hydrolysis will occur and generate unwanted MgOHCl: Generation of MgOHCl can be suppressed by dehydrating MgCl 2 ·6H 2 O in an HCl rich environment, but the corrosive action of HCl adds extra capital and operational costs to the process. Indirect methods have been proposed to prevent MgOHCl production, such as using ammonium chloride protections, gas protection heating, and organic solvent distillation. ,− However, each of these methods has certain disadvantages. In the gas protection heating method and the ammonium chloride protection heating method, MgCl 2 ·6H 2 O is protected by a cover layer of HCl gas and NH 4 Cl vapor, respectively.…”

“…This solution is heated so that the H 2 O is distilled off of the solution. Ammonia is bubbled through this biglycollate solution to yield magnesium chloride hexammoniate, which can be further calcined to produce anhydrous MgCl 2 . ,,, However, the thermodynamically delicate nature of the MgCl 2 –solvent–NH 3 (solvent = methanol, ethanol, or ethylene glycol) ternary system can be problematic in addition to the potential formation of a stable, unwanted ternary compound (MgCl 2 ·4CH 3 OH·2H 2 O). , Also, when the partial salt hydrate (2H 2 O or 4H 2 O) is ammoniated, the excess H 2 O in the solvent shifts the reaction to Mg­(OH) 2 formation. , These problematic reaction thermodynamics in existing Mg production methods have prompted the need for alternative methods for anhydrous MgCl 2 production.…”

“…Pownceby et al determined that when the ammonia (>17% supersaturated methanol + NH 3 and NH 3 bubbling) was added into the MgCl 2 · x H 2 O + methanol solution, unexpectedly, MgCl 2 ·6NH 3 did not form. If the feedstock contains anhydrous MgCl 2 + methanol, however, MgCl 2 ·6NH 3 will precipitate. , The presence of excess water favors the formation of magnesium hydroxide (Mg­(OH) 2 ) and ammonium chloride­(NH 4 Cl) in competition with the reaction to form hexammoniate (eqs and ). This unwanted NH 4 Cl is dominant given the presence of significant H 2 O in the system.…”

“…For the ammonium chloride protection heating method, there is also a very high operating condition (700 °C) required. In the case of the organic solvent distillation method, MgCl 18,22 The presence of excess water favors the formation of magnesium hydroxide (Mg(OH) 2 ) and ammonium chloride-(NH 4 Cl) in competition with the reaction to form hexammoniate (eqs 6 and 7). This unwanted NH 4 Cl is dominant given the presence of significant H 2 O in the system.…”